Traditional wooden frame structures are considered aesthetically pleasing but are not very robust, particularly not when exposed to moisture, and they lack the insulating properties wanted in modern buildings.
The robustness issues have been solved with window frames made from polyvinyl chloride (PVC), which is very widely used, while other window makers, including the applicant, have chosen to make the frames from profiles with a wooden core, typically of plywood, and a polyurethane (PUR) shell. Other plastic materials have also been used for the shell, but PUR is by far the most widely used, since it has suitable properties with regard to weather resistance, insulation, mouldability etc.
These frames, however, still lack sufficient insulating properties to live up to still stricter building regulations, which are being adopted in many countries, and it has therefore been attempted to include insulating materials in the frame structure. One example of this is the applicant's own prior patent application WO2007/057029, where the core is made at least partially of heat-treated wood, which has improved insulating properties; another, DE19516486A1, where the core is made from insulating foam.
In DE19516486A1 the preferred insulating material was foamed PUR covered by a PUR shell and it was explained that the shell and the core should preferably be made from the same material to ease recycling. No other materials were mentioned and practice showed that very few other commercially available materials are in fact suitable for use in the core, and that most of these are too costly for practical use.
As PUR too is a relatively expensive material, it is the object of the invention to provide an alternative and cheaper frame structure still having good thermal insulating properties.